Some librarians are probably now thinking they have a patriotic duty to see what people are reading and to report any "suspicious" behavior. Part of the intent of the Patriot Act and the Library Awareness Program was to bamboozle the nation's librarians into acting as the kind of "ward watchers" that were once so common in the Soviet Union (the babushkas who sat on each floor of apartment buildings and filed reports on the comings and goings of their flock).
The purpose of this is purely a show and indoctrination. 1. No self-respecting terrorist would go to a fucking library to do terror reading (maybe there is something positive here - I think that we should get protected by pigs from extremely dumb terorists.) 2. No library that I am aware of requires ID to do on-site anything. The same goes for internet cafes and open wireless access points. 3. Buying books for cash is anonymous as it gets - it's unlikely that a library will have something that university bookstore or Internet doesn't have. Again, poor terorists should be caught. I want to be terorized by professionals.